The invention relates to a spacer of a fuel assembly for a light-water cooled nuclear reactor, according to European Patent Application 0 027 203 A1, corresponding to U.S. Pat. No. 4,578,239. The invention also relates to a spacer of such a fuel assembly with a pair of springs held on a common web, according to U.S. Pat. No. 3,679,546.
In fuel assemblies which are subjected to a strong cooling water flow, fuel rods that are set into powerful vibrations are therefore laterally supported in meshes of grid-shaped spacers, in order to damp those vibrations, exclude mechanical damage and hold the rods at a distance apart which is technically safe thermally. Those spacers frequently have square meshes which are formed from crossed webs and contain springs that laterally support fuel rods respectively inserted in the meshes.
A spacer formed of crossed webs is known from U.S. Pat. No. 3,679,546, in which each fuel rod of a mesh is supported by a spring formed of a long sheet-metal strip that extends parallel to a center line of the fuel rod. That sheet-metal strip has an approximately C-shaped longitudinal section with a resilient or sprung central part which merges at the top and bottom (in relation to the center line direction of the fuel rod) through the use of a convex curvature into end strips that extend along and are supported by the inside of a web forming the mesh, facing toward the fuel rod. Assembly slots extending transverse to the fuel rods are respectively associated with the two end strips and the ends of the end strips are bent in such a way that they reach through the assembly slots and are still in contact with the back of the web.
In the working position, each spring is located approximately in the center of a mesh side. The spring can be displaced laterally (i.e. along the slots) from the operating position into an assembly position. The assembly slots merge at that position into enlarged cross-sectional areas so that the bent ends of the end strips can be inserted in or extracted from the enlarged cross-sectional areas.
In order to prevent an unintentional displacement, the convex curves of the springs are configured as protrusions which, in the operating position, protrude into corresponding recesses on the upper edge and lower edge of the web but which, for assembly and dismantling, have to be bent away from the wall of the web.
Particular spring materials are necessary in order to produce the spring force necessary for optimum support of the fuel rods. However, those materials frequently have a high absorption for neutrons which are necessary in the reactor in order to maintain nuclear fission. Although relatively little material is necessary for those known springs, the spring forces which are necessary can only be achieved by special materials.
A spring is known from European Patent Application 0 527 244 A1 (corresponding to U.S. Pat. No. 5,311,564) which likewise has a C-shaped cross section but is configured as a xe2x80x9cdiagonal springxe2x80x9d. In other words, it does not protrude into the mesh from a side surface of the mesh and at right angles to the web but instead diagonally from a corner of the mesh at which two webs cross. For that purpose, arms (end strips) of the C-shaped spring have long straps extending at right angles to the fuel rods on both sides and the two straps are formed practically into the shape of a right-angle triangle which can be inserted in the corner of the mesh. All of the straps pass through corresponding assembly openings in one of the two webs. They are held in the assembly openings of that one web and are in contact with the other web.
As compared with the spring of U.S. Pat. No. 3,679,546, it is in fact an advantageous feature that two such diagonal springs can be combined to form a double spring which protrudes into adjacent corners of two meshes located side by side. However, the way in which the springs are fastened to the mesh walls does not, in both cases, permit two such springs to be disposed in the center of a web in such a way that the two springs protrude into two adjacent meshes from the common web.
It is shown in European Patent Application 0 027 203 A1 that a fastening of straps on the rims of the end strips, similar to European Patent Application 0 527 244 A1, is also suitable for one spring which is held on a web in the center between two corners of a mesh. That spring has the features of the prior art mentioned initially above.
Those fastening straps require a relatively large amount of material which can be disadvantageous with respect to the neutron absorption as well as the natural frequency of the vibrating springs.
A similar spring with two end strips, which respectively engage in slots of a web on each side through the use of profiled sections but with which less material is required for the straps, is also presented in International Publication No. WO 94/09495, corresponding to U.S. Pat. No. 5,539,792. That spring is not associated with structural limitations with respect to the shape of the spring. Instead, the fastening is so flexible that the spring shape can be matched to the respective requirements of the reactor. In particular, therefore, the C-shape known from other documents can also be retained. However, the use of that fastening principle does not appear to be suitable for spring pairs in which the springs of the pair respectively protrude in opposite directions into the meshes, on a common web between two adjacent meshes.
It is also a disadvantage that the springs for those structures are relatively loosely seated in the openings in which their end strips are seated because it has to be easy to insert them during assembly. However, they should not be able to fall out even when no fuel rods have as yet been inserted, in the ready-for-use condition. In addition, during operation of the reactor, they should not be shaken by the cooling medium flow to such an extent that mechanical damage to the springs or the fuel rods can occur.
It is accordingly an object of the invention to provide a spacer with specially fastened springs for fuel assemblies of nuclear reactors, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and in which the springs have a retention on a web of the spacer that involves little material, leaves a choice of a favorable shape of the springs practically unlimited and is seated as well as possible.
With the foregoing and other objects in view there is provided, in accordance with the invention, a spacer of a fuel assembly for a light-water cooled nuclear reactor, comprising:
webs crossing each other to form meshes for inserting fuel rods having center line directions, the webs having snap-in openings formed therein, the webs having assembly openings formed therein with substantially slot-shaped opening parts having ends with edges, the assembly openings having enlarged opening parts each leading from a respective one of the substantially slot-shaped opening parts in parts of the webs, and the webs having inner surfaces facing toward the fuel rods;
a plurality of springs each held in one of the assembly openings in one of the webs, the springs having an operating position and an assembly position offset laterally relative to the operating position, and the springs having a spring force;
each of the springs having a resilient central part facing toward a fuel rod, the central part having a top and a bottom relative to the center line direction, and the central part having end strips each extending substantially parallel to a respective one of the webs and adjoining a respective one of the top and bottom; each
of the end strips having two lateral rims each extending parallel to the center line direction and bent away from a fuel rod about the center line direction;
profiled sections each seated on a respective one of the rims of the end strips, the profiled sections having inner parts respectively supported on the inner surface of a respective one of the webs in the operating position of the spring, and the profiled sections having outer parts encompassing the edges at the ends of the opening parts through the substantially slot-shaped opening parts;
the outer parts of the profiled sections to be inserted in the enlarged opening parts in the parts of the webs corresponding to the assembly position of the spring and displaced from the assembly position along the slot-shaped opening parts into the operating position; and
the springs having at least one part pressed by the spring force into the snap-in opening and/or the central parts of the springs and/or at least one of the end strips carrying a protuberance pointing away from a fuel rod and engaging in the snap-in opening, in the operating position of the springs with the fuel rod inserted.
In this spacer, the spring has a resilient central part which faces toward the fuel rod and on which respective end strips are adjoined at the top and the bottom, relative to the center line direction of the fuel rod. In accordance with U.S. Pat. No. 3,679,546, only the narrow rim, disposed approximately at right angles to the fuel rod, of the upper and lower end strips is bent to form a profiled section which is seated in the corresponding assembly opening and encompasses the rim of the web. However, in accordance with the invention each of the two end strips has two laterally protruding rims which are bent to form profiled sections and there are, therefore, four profiled sections for holding the spring. The rims therefore extend approximately parallel to the center line direction and are bent away from the fuel rod.
The profiled sections seated on the rims of the end strips are supported, in the assembled condition of the spacer, i.e. in the operating position of the spring, on an inner surface of a web forming the mesh and facing toward the fuel rod through the use of an inner part of each profiled section. An outer profiled section part of each profiled section protrudes through an essentially slot-shaped opening part of an assembly opening of the web. This outer profiled section part encompasses the edge of the web at the end of the respective slot.
Four slot-shaped opening parts of the assembly opening can then be provided corresponding to the four profiled sections. These opening parts lead to an enlarged cross-sectional area of the assembly opening, which is located at a position of the web which is offset laterally relative to the operating position of the spring. The spring is inserted at this laterally offset position (xe2x80x9cassembly position of the springxe2x80x9d) and each slot-shaped opening part therefore leads to an enlarged opening part which is shaped in such a way that the outer parts of the profiled sections, which are located on the sides of the end strips, can be inserted into the assembly opening in the assembly position of the spring. The spring which is inserted in this way can then be displaced along the slot into the operating position.
As the figures also make clear, four enlarged opening parts that are separated from one another can therefore, in particular, be provided. The enlarged opening parts are seated at the end of the four slot-shaped opening parts so that four spatially separated openings occur and form the assembly opening. It is, however, also possible for slot-shaped opening parts located one above the other to open into a common enlarged opening part. It is likewise also possible to provide for respectively associating the rims of the upper end strip with an upper common slot and the rims of the lower end strip can also engage in a common lower slot, through the use of their profiled sections.
In accordance with a special configuration of the invention, the assembly opening can then itself have a widening, i.e. an enlarged opening area, that represents a snap-in opening in which a protuberance on the central part of the spring or one of its two end strips engages and points away from the fuel rod. It is, however, also possible to provide its own snap-in opening in the web. In addition, a convex curvature of the ends of the central part or a similar part of the spring can be used instead of the protuberance for engagement of the spring. This part or the convex curvature is positively pressed into the corresponding snap-in opening as soon as the spring is seated in its assembly position and a fuel rod is inserted.
With the objects of the invention in view there is also provided a spacer of a fuel assembly for a light-water cooled nuclear reactor, comprising:
webs crossing each other to form first and second meshes for inserting fuel rods having center lines, the webs including common webs adjacent two of the meshes, the common webs having a first side surface facing toward a first one of the fuel rods and a second side surface facing toward a second one of the fuel rods, the common webs having assembly openings formed therein defining rims of the assembly openings and web edges at the rims, the assembly openings having a widened opening area at a given position on one of the webs, at least one substantially slot-shaped opening part having opening edges extending transverse to the fuel rods and end edges disposed one above the other, and the webs having snap-in openings; and
first and second springs held in the assembly openings and having central parts, assembly positions, operating positions and spring forces;
a) the first spring including:
i) a first resilient central part protruding into one of the first meshes, the first resilient central part laterally supporting a first fuel rod to be inserted in the first mesh, and the first resilient central part having a top and a bottom relative to the center line of a fuel rod and two first end strips each adjoining a respective one of the top and bottom;
ii) four rims including left and right lateral rims disposed on each of the two first end strips, the four rims extending approximately parallel to the center line of the fuel rod and bent away from the first fuel rod;
iii) first profiled sections each disposed on a respective one of the four rims, each of the first profiled sections having an inner profiled section part supported on the first side surface of the common web in the operating position of the first spring, and an outer profiled section part encompassing the web edges through the assembly openings;
b) the second spring including:
i) a second resilient central part protruding into one of the second meshes, the second resilient central part laterally supporting a second fuel rod to be inserted in the second mesh, and the second resilient central part having a top and a bottom relative to the center line of a fuel rod and two second end strips each adjoining a respective one of the top and bottom;
ii) four rims including left and right lateral rims on each of the two second end strips, the four rims extending approximately parallel to the center line of the fuel rod and bent away from the second fuel rod; and
iii) second profiled sections each disposed on a respective one of the four rims, each of the second profiled sections having an inner profiled section part supported on the second side surface of the common web in the operating position of the second spring, and an outer profiled section part encompassing the web edges through the assembly openings;
c) the end strips including mutually opposite upper end strips and mutually opposite lower end strips, and one of the first and an adjacent one of the second profiled sections shaped to form four profiled section pairs, in the operating position of both of the springs;
i) the outer profiled section parts of the first profiled sections to be inserted from the first side surface and the outer profiled section parts of the second profiled sections to be inserted from the second side surface, in the widened opening area in the position on the one web laterally displaced relative to the operating position of the springs;
ii) the outer profiled section parts guided in the opening edges upon laterally displacement of the springs from the assembly position into the operating position; and
iii) a pair of the profiled section pairs located one above the other and positioned on the end edges upon jointly pushing the springs laterally into the operating position; and
one of the springs having at least one part pressed by the spring force into the snap-in opening and/or the central part and/or at least one of the end strips of each of the springs carrying a protuberance pointing away from a fuel rod and engaging in the snap-in opening, in the operating position of both of the springs.
In this embodiment, two springs are held in an assembly opening of a web which is common to two adjacent meshes, wherein the two springs are essentially similar in construction.
The spring has a resilient central part, which protrudes into the respective mesh and laterally supports the fuel rod inserted there. An upper and a lower end strip adjoin this central strip at the top and the bottom, relative to the center line of the fuel rod. Each of the two end strips has a left-hand and right-hand lateral rim which respectively extends approximately parallel to the center line of the fuel rod and is bent away from the fuel rod. This produces four rims, with each rim carrying its own profiled section. In the operating position of the spring, an inner profiled section part of each profiled section is supported on a side surface of the common web, facing toward the fuel rod, and an outer profiled section part reaches through the assembly opening. The web edges formed by the rim of the assembly opening are therefore at least partially form-lockingly encompassed by the inner part and outer part of each profiled section. A form-locking connection is one which connects two elements together due to the shape of the elements themselves, as opposed to a force-locking connection, which locks the elements together by force external to the elements.
In the assembled condition of the spacer (operating position of both springs), the springs are located on both sides of the web and, in fact, the upper end strips are located opposite to one another and, likewise, the lower end strips are located opposite to one another. They can then be in direct contact with the side surfaces of the web or they can also be held at a small distance from the web surfaces, for example in order to generate a slight angle between the end strips and the web.
The profiled sections are then shaped in such a way that each profiled section is located at the rim of one end strip of one spring adjacent a profiled section at the rim of the opposite end strip of the other spring. Therefore, a total of four profiled section pairs are present, corresponding to the two rims of the two end strips of each spring.
In this embodiment as well, each spring is inserted in an assembly position and is only displaced into the operating position after the insertion along slot-shaped opening parts. A widened cross-sectional area, in which all of the outer profiled section parts of both springs are inserted, on the surfaces of the web, then corresponds to the assembly position. The profiled section of one spring is then introduced from one side and the profiled section of the other spring is introduced from the other side into the widened cross-sectional area. This produces a spring combination with four pairs of adjacent profiled sections (namely left upper, right upper, right lower, left lower). In order to then push the spring combination from the assembly position into the operating position, the assembly opening contains at least one essentially slot-shaped opening part. Therefore, an upper edge is present which extends transverse to the fuel rods and which is used as a guide for the two upper profiled section pairs, while a lower edge of the opening part extending transverse to the fuel rods serves as a guide for the two lower profiled section pairs.
An upper end edge and a lower end edge at the end of the slot-shaped opening part extend approximately parallel to the center line of the fuel rod, are located practically one above the other and on which an upper profiled section pair and a lower profiled section pair are then in contact as soon as the springs are jointly displaced laterally into the operating position.
In accordance with a concomitant feature of the invention, the operating position of the springs is located on the web in the center between two adjacent corners of the mesh. At least one spring part is held by the force of the springs in a snap-in opening of the web, and the spring therefore is snapped into this position, at least in the loaded condition.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a spacer with specially fastened springs for fuel assemblies of nuclear reactors, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.